Standardize grouping of package imports

[laser]

Description

There exist a few different styles in the codebase today (see here and here). We should come up with a set of rules for how imported packages grouped together and enshrine those rules into our contribution doc.

I propose:

import (
        [stdlib packages, alpha-sorted]
        <single, empty line>
        [gx-installed packages]
        <single, empty line>
        [go get-installed packages (not part of go-filecoin)]
        <single, empty line>
        [go-filecoin packages]
)

Example:

import (
	"context"
	"testing"

	cmds "gx/ipfs/QmYMj156vnPY7pYvtkvQiMDAzqWDDHkfiW5bYbMpYoHxhB/go-ipfs-cmds"
	"gx/ipfs/QmcZfnkapfECQGcLZaf9B79NRg7cRa9EnZh4LSbkCzwNvY/go-cid"
	ipld "gx/ipfs/Qme5bWv7wtjUNGsK2BNGVUFPKiuxWrsqrtvYwCLRw8YFES/go-ipld-format"

	"github.com/stretchr/testify/assert"

	"github.com/filecoin-project/go-filecoin/testhelpers"
	"github.com/filecoin-project/go-filecoin/types"
)

[dignifiedquire]

I think we already agree on this, so 👍 from me for making it explicit

[phritz]

can i make my editor do this automatically? i use goimports on save which fixes things up but would love an additional hook. seems low pri but thought i'd ask.